If you see a printer in the list it’s because RMTT has run a script to secure against a MS vulnerability that is not patched as of July 1, 2021.
When this script run, you will not be able to install or modify printers unless you reverse the script. This can be done by running %RMTT%\Tools\Printers-Remove-ACL-Lock.ps1 from an elevated Powershell console.
call 303.732.3200 with any questions